1. Field of the Invention
The present invention relates to a sensorless method and related device for starting a three-phase brushless direct-current motor, and more particularly, to a sensorless method and related device capable of enhancing probability of successfully starting the three-phase brushless direct-current motor without a synchronization procedure.
2. Description of the Prior Art
Nowadays, brushless direct-current motors are widely used, and have several advantages, including small size, light weight, simplicity of design, and efficiency. Brushless direct-current motors are often utilized for fan motors, and spindle motors of storage apparatuses in electronic devices, such as personal computers, notebooks, communication devices, and home appliances. Generally speaking, position of a rotor needs to be detected while driving the brushless direct-current motors in order to drive a switch for commutation procedures.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a brushless direct-current motor system 10 in the prior art. The brushless direct-current motor system 10 includes a three phase brushless direct-current motor 102, a Hall sensor 104, and a driver 106. The three phase brushless direct-current motor 102 includes a rotor 108. The Hall sensor 104 has three Hall elements 110, 112, 114 for sensing position of the rotor 108. The driver 106 includes a logic unit 116 and a commutation switch 118. The logic unit 116 transmits a control signal to the commutation switch 118 according to the position of the rotor 108 detected by the Hall sensor 104 so as to drive the rotor 108. However, sensing accuracy of the Hall sensor 104 is easily affected by surroundings, even such that the Hall sensor 104 can not work in some environments (such as high temperature environments). On the other hand, the Hall sensor 104 may increase system volume and manufacturing cost of the brushless direct-current motor system 10. Therefore, a sensorless driving method without a sensor is introduced.
Please refer to FIG. 2 and FIG. 3. FIG. 2 is an equivalent circuit diagram of the brushless direct-current motor system 10 shown in FIG. 1. FIG. 3 is a driving time sequence diagram of the brushless direct-current motor system 10 shown in FIG. 1. T1, T2, T3, T4, T5, and T6 respectively represent time sequence of each switch of the commutation switch 118. As shown in FIG. 2 and FIG. 3, driving order for the switch 118 is 1-6, 1-2, 3-2, 3-4, 5-4, 5-6 in turn. Two phases are conducted and a third phase is floating, and Va, Vb, and Vc are corresponding voltage levels respectively, so that back electromotive force of the three phase brushless direct-current motor 102 can be detected through the floating third phase. As a result, position information of the rotor 108 is obtained by detecting a zero crossing point Z of the back electromotive force. Nevertheless, using the sensorless method, the back electromotive force of the three phase brushless direct-current motor 102 cannot be effectively detected at a low rotational speed or in a static state, and the position of the rotor also can not be obtained accurately, so that the driver 106 can generate a control signal to drive the three phase brushless direct-current motor 102. To alleviate the abovementioned problem, a starting mode must be added in the sensorless method for the rotor 108 to reach a specific speed so as to detect the zero crossing point of the back electromotive force accurately and generate the control signal for driving the motor. Please refer to FIG. 4. FIG. 4 is a schematic diagram of a starting procedure 40 of the brushless direct-current motor system 10 in the prior art. The starting procedure 40 comprises the following steps:
Step 400: Start.
Step 402: Set an initial control signal and an initial speed.
Step 404: Determine whether a synchronization time has passed. If yes, go to Step 406; otherwise, implement Step 404 again.
Step 406: Determine a synchronizing speed. When the synchronizing speed is greater than a predetermined speed, go to Step 410; otherwise, go to Step 408.
Step 408: Commutate a control signal of the brushless direct-current motor system 10, and increase the synchronizing speed.
Step 410: Switch the brushless direct-current motor system 10 to a sensorless mode.
Step 412: Detect the zero crossing point Z of the back electromotive force in order to determine a starting result.
Step 414: End.
According to the procedure 40, the prior art sets an initial control signal and an initial speed to accelerate the rotor of the brushless direct-current motor system 10. Then, it is determined whether the synchronization time has passed and the synchronizing speed is greater than the predetermined speed in order to decide to switch to the sensorless mode. Again, if no zero crossing point Z of the back electromotive force is detected, there is a starting failure, and the brushless direct-current motor system 10 must re-implement the starting procedure. However, the abovementioned starting method is unable to predict initial position of the rotor in practice. If the rotor and the initial control signal produce zero torque, i.e. magnetic field of a stator is oriented 0 degrees or 180 degrees with magnetic field of the rotor in the brushless direct-current motor, this will make the rotor unable to accelerate to the predetermined speed after the synchronization time has passed, and increases the possibility that no zero crossing point of the back electromotive force is detected after switching to the sensorless mode, causing starting procedure failure.